Add the ability to populate State from metadata

[tonyandrewmeyer]

The State could have some default components based on the Context metadata, or perhaps there could be a fill_from_context method or similar.

For example, this could add:

• A Container for each container in the metadata (although you'd probably want to be able to specify whether can_connect should be True or False, or it seems like there might be a lot of awkward dataclasses.replace afterwards)
• config that's the same as the defaults in the metadata
• A default Network (there's essentially always a Juju network, right?)
• A Storage for each storage in the metadata (to the minimum side of the multiple value if provided)
• A StoredState for each ops.StoredState attribute on the Context's charm class (not perfect since these can exist elsewhere, particularly in libs, but maybe a useful base?)

I don't think it could have relations (even if we do make it possible to have data-inaccessible relations, e.g. #1452) since there's a practical difference between a relation existing (from the point of view of the unit) and not existing, wider than just the data being available.

I don't think it could include resources, even though they're in the metadata and always (?) available, because it doesn't seem like there's any way to have a sensible default location for the content.

See this Matrix discussion for more background.